The present invention relates to a single crystal pulling apparatus having a plurality of shield blades capable of sliding motion for properly controlling the area of opening around a single crystal being pulled from a melt in the progress of crystal growth.
Czochralski method is one of the representative methods for the growth of single crystal from a melt. In Czochralski method, a melt is prepared in a crucible by melting a starting material for a single crystal. A seed crystal is brought into contact with the melt, so that a single crystal is grown on the seed crystal and pulled up from the melt.
During pulling the single crystal, components evaporated from the melt are likely to be condensed. The condensed components adhere onto various devices or parts incorporated in the apparatus, or drop in the melt. The dropping causes the inducement of various defects, e.g. dislocation and heterogeneity, in the growing single crystal.
For instance, volatile SiO is formed by the reaction of Si melt with a quartz crucible during pulling a Si single crystal. The formed SiO is condensed and deposited on the surface of internal devices at a relatively low temperature or on the surface of a growing Si single crystal. SiO deposited at the low-temperature part of the internal device would be dropped in the Si melt due to certain physical or thermal impacts. The dropping causes the fluctuation of the Si melt in the distribution of concentration or temperature, so that defects are induced into the growing Si single crystal. On the other hand, SiO deposited on the surface of the growing Si single crystal causes the inducement of defects, e.g. dislocations and strains, originated in the mismatching of crystal orientation.
The formation of defects derived from the condensation of evaporated components such as SiO is inhibited by providing a cover above the melt.
However, the formation of defects is caused by CO evaporated from carbonaceous internal devices and gaseous compoents residing in the atmosphere above the melt, too. These harmful components are removed from the crystal growth zone by the flow of inert gas. SiO, CO and other components which would have harmful influences on the growth of a single crystal are accompanied with the flow of the inert gas and discharged to the outside.
For instance, Japanese Patent Publication 57-40119 discloses the method wherein a cover is located above the surface of a melt and inert gas is let flow through a space between the cover and the surface of the melt. According to this method, a crucible 2 is rotatably supported with the rotary shaft 1 capable of vertical motion, and a screen 3 is located above the crucible 2, as shown in FIG. 1.
The screen 3 has an annular projection 3c extending through a connector part 3b from an annular rim 3a. The annular projection 3c is faced to the surface of a single crystal 4 being pulled up from a melt 4a. In response to the growth of the single crystal 4, a material 4b is supplementally fed to the crucible 2 through a heated quartz tube 5a passing through a side wall 5, to compensate for the melt 4a consumed in the growth of the single crystal 4. The melt 4a in the crucible 2 is held at a proper temperature by a heater 6 provided around the crucible 2.
A seed crystal 4c is attached to the lower end of a pulling shaft 7. The pulling shaft 7 is carried upwards in response to the growing speed of the single crystal 4. Hereon, inert gas such as Ar or He is introduced into a space above the screen 3. The inert gas flows through a gap g.sub.1 between the single crystal 4 and the annular projection 3c into a space above the crucible 2 covered with the screen 3. The inative gas successively flows through a gap g.sub.2 formed at the edge of tile crucible 2, a gap g.sub.3 between a heat insulator 8 and the rotary shaft 1 and a gap g.sub.4 between the heat insulator 8 and the heater 6 and then to the outside. This inert gas flow accompanies SiO evaporated from the melt 4a, CO evaporated from carbonaceous internal devices and other harmful ingredients to the outside.
The inducement of defects in an obtained sigle is inhibited to some extent by the flow of the inert gas to remove harmful ingredients from the crystal growth zone. However, the inner edge of the annular projection 3c for defining the gap g.sub.1 is located at a constant position, since the screen 3 is a unitary body made of SiC, carbon or the like. On the other hand, the single crystal 4 being pulled from the melt 4a changes its diameter in the course of crystal growth, so that the gap g.sub.1 is changed during the crystal growth. Concretely, the gap g.sub.1 from the inner edge of the annular projection 3c to the surface of the single crystal 4 is large at the initial stage of the crystal growth, while the gap g.sub.1 is held at a constant value when the crystal growth continues in a stationary state.
Since the gap g.sub.1 is usually determined on the assumption that the single crystal grows in the stationary state, the gap g.sub.1 is too large at the initial stage of crystal growth. The large gap causes the unexpected flow of inert gas at the initial stage, and SiO, carbon or the like adherently condensed on the internal devices is not effectively prevented from dropping onto the surface of the melt 4a. As a result, defects such as dislocations are likely to be induced into the single crystal 4 at the initial stage.
The unitary screen 3 is effective for manufacturing a single crystal 4 having a constant diameter. However, when a single crystal 4 different in diameter is to be manufactured, the gap g.sub.1 is changed in response to the diameter. The changed gap g.sub.1 is not suitable for the formation of a predetermined gas flow. In this sense, it is necessary to prepare a plurality of screens 3 each different from the other in diameter corresponding to the diameter of each single crystal to be manufactured.
Besides, the amount of the melt 4a in the crucible 2 is reduced as the growth of the single crystal 4. The consumption of the melt 4a causes the reduction of the contact surface between the melt 4a and the quartz crucible 2, so that the concentration of oxygen in the single crystal 4 becomes less from the intilal stage to the final stage of the crystal growth.
In order to keep the concentration of oxygen in the growing single crystal 4 at a predetermined value, the crucible 2 is rotated at a higher speed in the progress of the crystal growth to increase the amount of oxygen dissolved from the quartz crucible 2 into the melt 4a. However, the increase of the rotation speed changes the convection flow of the melt 4a in the crucible 2. Consequently, the gradient of a temperature and the concentration of oxygen in the melt 4a at the final stage differs from those at the initial stage, causing the inducement of defects into the single crystal.